Development is the process through which an embryo becomes an adult organism and eventually dies. Through development, an organism's genotype is expressed as a phenotype, exposing genes to the action of natural selection.

Studies of development are important to evolutionary biology for several reasons:

Explaining major evolutionary change
Changes in the genes controlling development can have major effects on the
morphology of the adult organism. Because these effects are so
significant, scientists suspect that changes in developmental genes have helped
bring about large-scale evolutionary transformations. Developmental changes may
help explain, for
example, how some hoofed mammals evolved into ocean-dwellers, how water plants
invaded the land, and how small, armored invertebrates evolved wings.

Mutations in the genes that control fruit
fly development can cause major morphology changes, such as two pairs of
wings instead of one.

Another developmental gene mutation can
cause fruit flies to have legs where the antennae normally are, as shown
in the fly on the right.

Learning about evolutionary history
An organism's development may contain clues about its history that biologists
can use to build evolutionary trees.

Characters displayed by embryos such as these
may help untangle patterns of relationship among the lineages.

Limiting evolutionary change
Developmental processes may constrain evolution, preventing certain characters
from evolving in certain lineages. For example, development may help explain why
there are no truly six-fingered tetrapods.